6-Bromo-2-naphthalenecarboxylic acid (referred also to as “BNA”, hereinafter) is used as an intermediate raw material for medicines and agrichemicals. As medicines and agrichemicals are such that impurities are particularly undesirable to be mixed therein, highly-pure materials with reduced impurity concentration as much as possible are also required for the intermediate materials to be used for those purposes.
Patent Document 1 below discloses a production method for the above BNA, in which 6-bromo-2-methylnaphthalene is oxidized by molecular oxygen in a solvent under the presence of an oxidative catalyst thereby to produce BNA, wherein the solvent contains lower aliphatic carboxylic acid and the catalyst comprises a heavy-metal compound and a bromine compound.
However, when BNA is produced under the condition specifically described in the above literature, problems occur including that unignorable impurities remain and the refinement thereof is not easy.
More specifically, Patent Document 1 recommends, as a preferred condition for the oxidation reaction, to employ a reaction temperature of 120 to 200 degrees C. and a reaction pressure of 10 to 30 kg/cm2 (0.98 to 2.94 MPa). A specific example employs a reaction condition of 175 degrees C. and 30 kg/cm2. Such a reaction condition was considered to be indeed satisfactory with regard to the productivity, but the purity of BNA in the obtained product was poor. Accordingly, in order to provide a product with high purity of BNA, advanced refining treatment was necessary to be performed for the product obtained through the above method. In addition, another problem was the high cost for the necessity of facilities comprising special materials because of the above oxidation reaction being executed under a high pressure.
The present inventors searched for a means for solving the above problems. As a result, a method has been found out which produces relatively pure BNA with a high yield by modifying conditions for oxidation (Patent Document 2). As disclosed in Patent Document 2, the oxidation reaction for 6-bromo-2-methylnaphthalene is executed under a lower temperature and a lower pressure than those disclosed in Patent Document 1 thereby to allow for enhancing the purity of BNA in the product.